1. Field of the Invention
The present invention relates to a lithography process for use during the production of semiconductor integrated circuits. More particularly, it relates to the removal of chlorine or bromine which remains on a surface of a conductive film when the conductive film made of aluminum or an alloy thereof is dry-etched by using chlorine or bromine, or an alloy thereof as an etchant.
2. Description of the Related Art
For a wiring forming a semiconductor integrated circuit formed on a substrate, such as silicon wafer or the like, thin films of aluminum (Al) or thin films of an alloy in which silicon (Si) or copper (Co) is added into aluminum are often used. To prevent an increase in the contact resistance of an aluminum or aluminum alloy thin film wiring due to an alloy reaction with a silicon wafer, a so-called barrier metal of a thin film of titanium (Ti), titanium nitride (TiN), or titanium-tungsten (TiW) is provided between the silicon wafer and the aluminum thin film.
Patterning of an aluminum or aluminum alloy film for such wiring as described above is performed generally by a lithography in which the conductive film is selectively etched by using a mask formed of a resist layer. Anisotropic etching is required to make it possible to form fine wiring patterns. At the present time, reactive ion etching (RIE) is a typical anisotropic etching method. For removing a resist mask, a so-called ashing, which can be performed Without using a solvent, such as trichloro ethylene which poses a problem relating to the environmental pollution, is used.
The above-mentioned etching and ashing methods are both a dry process. So, they are suitable for process control or automatic processing and free from the contamination due to impurities in an etching solution or a solvent as in a wet process. An outline of these processes will now be explained with reference to FIGS. 1 and 2.
FIGS. 1(a), 1(b), and 1(c) show a change in the cross section of a member to be processed in the above-described dry etching and ashing processes. FIG. 2 schematically shows an example of the construction of a processing system for automatically performing the etching and ashing operations.
In the system in FIG. 2, a RIE apparatus 10 for etching aluminum films and an ashing apparatus 20 for removing resist masks after etching are connected to each other via a load lock chamber 13 which is capable of a vacuum. Aluminum films are transported by the load lock chamber 13 from the RIE apparatus 10 to the ashing apparatus 20 without contacting the atmosphere. Another load lock chamber 13A is disposed on the entry side of the RIE apparatus 10, and another load lock chamber 13B is disposed on the exit side of the ashing apparatus 20. Substrates on which aluminum films are formed can be inserted into or taken out of the RIE apparatus 10 and the ashing apparatus 20 without introducing air into the apparatuses 10 and 20 by the load lock chambers 13A and 13B.
Referring to FIG. 1(a) , for example, an aluminum film 2 is deposited on the whole of a surface of a substrate 1 composed of a silicon wafer, following which a resist is applied onto the aluminum film 2. By applying ultra-violet rays, electron beams, or an energy beam, such as excimer laser, or the like, to a predetermined position of this resist and then developing, a mask 3 composed of the aforesaid resist is formed. The surface of the substrate 1 on which the aluminum film 2 is formed is generally covered with an unillustrated insulation layer composed of SiO.sub.2, etc. The surface of the substrate 1 or a lower layer wiring is exposed inside a contact hole provided on a part of the insulation layer.
The substrate 1 with the mask 3 formed thereon as described above is placed on a stage 11 inside the RIE apparatus 10 through the load lock chamber 13A in FIG. 2. Then, for example, chlorine gas (Cl.sub.2) is introduced into the RIE apparatus 10 and, while the inside of such apparatus is being maintained at a predetermined pressure, a plasma is generated by applying a voltage between the stage 11 and an electrode 12. As a result, the aluminum film 2 is anisotropically etched, as shown in FIG. 1(b).
The substrate 1 having the aluminum film 2 etched as described above is transported to the ashing apparatus 20 through the load lock chamber 13 in FIG. 2. Then, for example, an oxygen gas (O.sub.2) is introduced into the ashing apparatus 20, and while the inside of such apparatus is being maintained at a predetermined pressure, a voltage is applied between a pair of electrodes 16 which are opposed to each other. As a result, a plasma is generated between the electrodes 16. The mask 3 composed of the aforesaid resist reacts mainly with oxygen atoms or molecules, or ions in this plasma and vaporizes, being exhausted outside the ashing apparatus 20. In this manner, the mask 3 on the aluminum film 2 is removed, as shown in FIG. 1(c).
Shown in FIG. 2 is the ashing apparatus 20 which performs plasma ashing in which a member to be processed is directly exposed to the plasma. The above-described processes are performed in the same manner as for an automatic processing system equipped with a so-called down-flow type ashing apparatus which exposes the member to be processed to only neutral active species extracted from a plasma. As a means for generating the aforesaid plasma, excitation by microwave radiation or excitation using a high-frequency induction coil is often used in place of the electrode 16.
In a RIE for films of aluminum or an alloy thereof, gaseous chlorine compounds, such as boron trichloride (BCl.sub.3) or silicon tetrachloride (SiCl.sub.4), bromine gas (Br.sub.2), or gaseous bromine compounds, such as hydrogen bromide (HBr) or boron tribromine (BBr.sub.3), are also used as an etchant.
If the substrate 1 upon which etching and ashing has been performed as described above is taken out into the atmosphere, a phenomenon is often recognized in that "after-corrosion" occurs in wiring composed of thin films of aluminum or an alloy of aluminum. The resistance of the wiring increases due to this after-corrosion, and in extreme cases disconnection occurs. Such after-corrosion proceeds while a semiconductor integrated circuit in a state in which the wiring is covered with a passivation insulation layer is used for a long period of time, thereby resulting in the poor reliability of products.
The mechanism causing such after-corrosion as described above is not yet completely clarified. It is considered that after-corrosion is due to the fact that chlorine, bromine, or their compounds, which are components of the etchant used in etching can, remain on the surface of an aluminum film. That is, the residual chlorine, for example, reacts with the water of the atmosphere, generating hydrochloric acid (HCl), etc., which causes aluminum films to become corroded.
The introduction of the automatic processing system shown in FIG. 2 enables an aluminum film to be sent to an ashing apparatus without being exposed to the atmosphere. And, since most of the remaining chlorine or the like are removed by the ashing apparatus. Accordingly, after-corrosion described above is considerably reduced.
In recent years, however, aluminum-copper (Al-Cu) alloys, in which electro-migration and stress migration do not often occur in comparison with pure aluminum films, have come to be used as a wiring material. As mentioned earlier, thin films of Ti, TiN, or TiW are used as barrier metals for blocking the alloy reaction between a silicon substrate or polycrystal silicon lower-layer wiring, and aluminum wiring.
The use of Al-Cu alloy films or barrier metals promotes after-corrosion because an electric cell is formed on the grain boundaries of different types of metals or the interface of the barrier metal and aluminum films because of the presence of hydrochloric acid generated from the above-mentioned residual chlorine. Therefore, even if the automatic processing system shown in FIG. 2 is introduced, a problem is posed in that after-corrosion cannot be completely avoided.